The differential atomic response of the topmost graphene layer on graphite

The atomic response of the topmost graphene layer on graphite was studied by using scanning tunneling microscopy (STM) as a function of tunneling gap distance, gap voltage and bias polarity. The contrast of the site-dependent topographical image depends on the gap distance, and the site-dependent tunneling current order of magnitude at a given gap distance is switched with the gap voltage (i.e. the contrast is significantly altered). The site-dependent current order is altered at the lower positive gap voltage as the gap distance is reduced between the probe and the carbon atoms of the topmost graphene layer. The switching in the atomic image contrast and the current order of magnitude is directly related to the differential atomic response of carbon atoms to the STM probe originating from the electronically active and mechanically soft β-carbon atoms.